A power shift gear typically includes a number of solenoid valves. Each solenoid valve is actuated, i.e., opened and closed, by a respective electronic switch electrically coupled thereto. An electronic control apparatus including the respective electronic switches may be used to control the switching sequence of the solenoid valves in response to a position of a shift selector lever. In order to, for example, switch the solenoid valves of a power shift gear in a purposeful way, it is frequently not sufficient that the control apparatus actuates the individual solenoid valves purely depending on the position of a selector lever. Rather, it is advisable, and indeed sometimes absolutely necessary, to take into account the position of various information switches, which may be arranged at different positions outside the electronic control apparatus. The electronic control apparatus must, therefore, be capable of reading in the position of these information switches.
With a known control apparatus of the above-mentioned type, the control apparatus is connected via information lines to the individual information switches. Closing the information switches has the effect of applying a voltage to the information lines which is registered by the control apparatus. Major problems occur if the electronic control apparatus needs to be interchangable. In this case, at least one contact has to be provided at the connecting plug(s) for each information switch. This then plays a particular role if, with an existing electronic control apparatus, all contacts are in use and the number of information switches is to be increased. In this case it takes, as a rule, a great deal of technical effort to design a new plug or provide an additional plug respectively.
The electronic switches used in the known electronic control apparatus described here are known inter alia from the data sheet "Smart SIPMOS BTS 412 A" (1988, Siemens, B4, pp. 1/12 through 12/12). The status signal of these switches permits monitoring of electrical resistance loads through the control apparatus. In particular, with an open switch, an open, i.e. incorrectly connected load resistance, and with a closed switch, a short-circuited load resistance is detected, i.e. a potential source of danger for the supply voltage. For this information, no additional information lines between the load resistances and the control apparatus are necessary. Rather, said information can be obtained from the control apparatus via the corresponding load lines.
A circuit arrangement for the inquiry of switch positions is known from DE-US 40 15 271, whereby a multitude of information switches is provided. The information switches are on the one hand connected to a fixed potential and on the other hand via an information line to a microcomputer as well as a resistor. The side of the resistors facing away from the information switches can in turn be connected via switches controlled by the microcomputer to a potential which deviates from the fixed potential. In this manner, the voltage drop across the measururing instrument setup can be raised during the reading-in of the positions of the information switches in such a way that a signal, well above the noise, is registered by the microcomputer. On the other hand, linked with this, the occurence of a comparatively large leakage current is avoided for those time periods in which the position of the information switches is not read in. With the known circuit arrangement, on average more than one line stemming from the microcomputer is provided for reading-in from one switch.